This invention relates to non-conductive, flexible, substrates having a flexible, conductive coating and their use in gaskets, seals, and other articles.
There is a need for gaskets, seals, and other products that can seal enclosure cabinets and other electrical and electronic housings to provide protection against moisture and dust. Such gaskets are placed in covers and between frames, panels, and doors of electronic equipment, cabinets, and housings. These gaskets must be soft and flexible with low compression deflection values. They must be capable of being compressed at least 50% for long periods of time without taking a compression set. They must also maintain their conductive and compression recovery properties after many compression-relaxation cycles.
These gaskets and seals must also provide an air and water tight seal. Products currently available are harder than is desirable resulting in the necessity of high closure forces being needed to produce an adequate seal. Because of this, parts to be sealed need to be thick and rigid to withstand these high closure forces.
Gaskets with low compression deflection values will permit cost and weight savings because thinner and lighter materials can be used for the part to be gasketed.
It is often necessary, particularly in electrical and electronic applications, that the gasket provide EMI/RFI shielding. Hence, typical gaskets are made with a conductive material in order to provide continuous conductivity and EMI/RFI shielding.
Conductive plastic-based products are particularly desirable for gaskets due to their good performance characteristics and ease of manufacture. However, due to the high concentration of conductive metals, the cost of such conductive plastics is high. This is especially true when a conductive metal such as silver is used.
Most conductive fillers are noble metalsxe2x80x94metal coated with a metallic core or metal coated with a non-metal core. Most conductive fillers are hard and because the fillers must be used in high concentrations, the plastics tend to become hard, stiff and brittle compared to plastics that do not contain conductive materials. Currently available conductive gaskets are harder than non-conductive gaskets. It is also not possible to make them soft and flexible. They tend to be semi-flexible to rigid. Furthermore, conductive fillers tend to degrade the properties of the plastic matrix material in which they are incorporated. Therefore, the plastic binders need to be harder to hold the gasket or part together. This low binder concentration will cause the part to have poor physical properties.
A number of products have been developed to address the need for flexible, conductive gaskets. One product is a high frequency EMI/RFI shielding gasket made by wrapping a strip of knit mesh material or wire mesh around the exterior of a resilient core. Such a mesh-covered core is described in U.S. Pat. No. 4,652,695. The core can be made from any highly compressible material but is usually a flexible, non-conductive polyurethane or polyethylene foam. The wrap is tough and imparts good cut and abrasion resistance. The method of applying the wrap to the core is very efficient and less expensive than other available technology. Good shielding values are obtained, however, the wrap is stiff, causing high compression deflection values. The stiff wrap also makes it very difficult to bend the gasket. It is usually supplied in straight sections. Pieces have to be butted up against each other to form a continuous gasket. Other problems associated with this type of gasket are that the mesh itself usually contains large quantities of nickel or silver. This makes the wrap very expensive. It is not possible to make a waterproof seal. Water can leak in wherever the gasket sections are joined. Installation of these gaskets is very labor intensive causing the installation costs and therefore the final gasket costs to be high. Furthermore, even though the polyurethane foam may have good compression recovery, the wrap has very poor memory. This results in a gasket with poor compression recovery.
Until now, it has not been possible to apply a curable liquid conductive coating on a flexible substrate. There are a number of reasons for this. The polymeric binder and many of the conductive fillers used for EMI shielding are much harder than the desirable flexible substrates. Therefore, conductive coatings tended to be too hard. Flexing, elongating, or compressing the gasket resulted in cracks in the conductive coating. These cracks cause a deterioration of properties, including electrical conductivity. Many coatings that do not visually show signs of cracking will still lose conductivity on flexing or compressing. It is also difficult to bond to flexible substrates such as foams. If the conductive coating is not sufficiently flexible, applying it to a flexible substrate destroys the advantages of flexibility, softness and low compression deflection values.
There is a need for a soft, flexible, conductive coating without the disadvantages associated with the prior art.
This invention is directed to non-conductive substrates having conductive coatings. These coatings do not interfere with the good compression recovery of the substrate. Using this system, lower compression deflection values can be obtained which generate lower closing forces in gasket and seal applications.
The conductive coatings of this invention are easier to apply than other types of coatings. The coatings are soft and flexible, bond well to a variety of substrates, and are easily applied. In addition, the conductive coatings are less expensive than wrap and other available technology.
Products produced from conductive polymer coatings applied to flexible substrates retain their physical, mechanical and electrical properties under most usage conditions, such as repeated flexing, bending and stretching. Such products include gaskets. The finished gasket can be supplied on a continuous roll or it can be formed in place. Gaskets and seals can be made water and air tight.
The invention is directed to a non-conductive polymeric substrate having a conductive coating wherein at least one of the substrate and coating is a foam polymer and the other is a foam polymer or an elastomer, wherein the conductive coating has flexibility equal to or lower than the flexibility of the non-conductive substrate, and wherein the conductive coating comprises at least one conductive filler dispersed therein in an amount effective to provide EMI/RFI shielding.